Abstract: The Fula Subbasin, located in the northeast Muglad Basin, has an area of 5000 km2. The FN oilfield is the biggest oilfield discovered in this subbasin with 100 million tons of oil reserves. Its main hydrocarbon reservoirs included the Aradeiba, Bentiu and Abu Gabra Formations of the Cretaceous. Based on a systematic geochemical analysis of crude oil, the reservoirforming stages of the FN oilfield are discussed. The geochemical characteristics of the crude oil from the three formations are very similar, which indicates that all hydrocarbons originated from the same source rocks, i.e., the Abu Gabra source rocks. The crude oil has both normal oil characteristics, such as full series distribution of nalkanes, hopanes and steranes, as well as biodegraded oil characteristics, such as raised chromatogram baseline and appearance of sharp UCM and 25norhopanes. It revealed, therefore, that the FN oilfield had undergone two oil infilling stages, i.e., the earlier infilling oils had relatively low maturity and was strongly biodegraded, and the later ones had relatively high maturity. The tectonic, burial and thermal evolution histories were used to analyze the hydrocarbon generation and charging and accumulation stages of the FN oilfield. It was shown that the Abu Gabra source rocks experienced two periods of hydrocarbon generation. The first one took place during the Late Cretaceous, with the maturity (Ro) of Abu Gabra source rocks reaching 0.6% within the hydrocarbon generation window; then the crude oil suffered relatively serious biodegradation due to tectonic reversion and uplift by the end of the Cretaceous. The second one happened during the late Paleogene as the Abu Gabra source rocks entered the hydrocarbon generation peak stage. This was the most important process for hydrocarbon generation and oilfield formation in the region. The structural traps related to the second period are the main targets for hydrocarbon exploration in the Fula Subbasin.

Key words:  Muglad Basin, Fula Subbasin, FN oilfield, biodegradation, hydrocarbon generation stage, reservoir formation